1. Field of the Invention
This invention relates in general to helical cavity resonators and more particularly to tunable helical cavity resonators utilizing both inductive and capacitive tuning to simultaneously achieve wide tunable frequency range and low insertion loss.
2. Prior Art
In communications applications helical cavity resonators are well known and widely used circuit elements. They resonate at a frequency for which the shielded cavity and the length of the wire comprising the helix within the shield are substantially a quarter wave length long.
In the helical cavity resonator art the distributed inductance and capacitance which establish the resonant frequency are determined by the physical dimensions of the cavity, the helical coil and its support member. Therefore, to design a tunable helical cavity resonator for resonance over a particular frequency band, exact and precise construction is required. Every dimension in a helical cavity resonator is critical to some degree of exactness.
In the past, to tune a helical cavity resonator it was customary to vary either the inductive characteristics or the capacitive characteristics of the resonator. To provide a capacitively tunable helical resonator it is known to use a grounded metal screw inserted into the cavity in proximity of the high impedance end of the helix coil and secured by screw threads in the grounded shield. By adjusting the physical distance between the top of the helix conductive wire and the grounded metal screw, the capacitive characteristics of the cavity walls resonator can be changed.
An alternate method of capacitive tuning is to provide a dielectric cap which is movable relative to the wire helix and the resonator cavity walls, thereby displacing the air in the space between the helix and the cavity walls resulting in a change in distributed capacitance between the wire helix and the cavity walls.
For inductive tuning of a helical cavity resonator it is known that insertion of a metal slug within the hollow formed by the wire helix coil reduces the total inductance of the helix coil thus varying the inductance of the cavity as the slug is moved within the hollow. The change in inductance is caused by circulating currents induced in the slug by the wire helix which in turn induce a counter emf in the wire helix.
These prior art approaches have shortcomings from the standpoint of limited tuning range, difficulty in fabrication, poor tuning accessability and relatively high manufacturing costs. Moreover, their performance is inferior in other aspects and their use in communication equipment is limited. Tunable cavity resonators represent a considerable portion of the cost of radio apparatus, and hence ways are constantly being sought to make such resonators as inexpensive as possible especially where high volume manufacturing is involved. In view of these considerations, a need exists for a low cost helical cavity resonator with a wide tunable frequency range, low insertion loss and easy tuning.
It is the general objective of this invention to provide an improved helical cavity resonator that over comes the foregoing deficiencies.
More particularly, it is the object of this invention to provide a helical cavity resonator which has an increased usable frequency range.
It is another object of this invention to provide an assembly process for a helical cavity resonator that has only a few steps and results in an inexpensive, high yield construction.
It is still another object of this invention to provide a helical cavity resonator that contains a low loss extension piece attached to the tuning slug allowing for easy adjustment from outside the wire helix with the use of a suitable tool.